The present invention relates to a process for the electrochemical graining of aluminum for use as printing plate supports, the process being performed by means of an alternating current in an acid electrolyte containing .beta.-diketo compounds.
Printing plates (this term referring to offset-printing plates, within the scope of the present invention) usually comprise a support and at least one radiation-sensitive (photosensitive) reproduction layer arranged thereon, the layer being applied to the support either by the user (in the case of plates which are not pre-coated) or by the industrial manufacturer (in the case of pre-coated plates).
As a layer support material, aluminum or alloys thereof have gained general acceptance in the field of printing plates. In principle, it is possible to use these supports without modifying pretreatment, but they are generally modified in or on their surfaces, for example, by a mechanical, chemical and/or electrochemical roughening process (also called graining or etching in the literature), a chemical or electrochemical oxidation process and/or a treatment with hydrophilizing agents. In the modern, continuously working high-speed equipment employed by the manufacturers of printing plate supports and/or pre-coated printing plates, a combination of the aforementioned modifying methods is frequently used, particularly a combination of electrochemical graining and anodic oxidation, optionally followed by a hydrophilizing step.
Graining is, for example, carried out in aqueous acids, such as aqueous solutions of HCl or HNO.sub.3 or in aqueous salt solutions, such as aqueous solutions of NaCl or Al(NO.sub.3).sub.3, using an alternating current. The peak-to-valley heights (specified, for example, as mean peak-to-valley heights R.sub.z) of the grained surface, which can thus be obtained, are in the range from about 1 to 15 .mu.m, particularly in the range from 2 to 8 .mu.m. The peak-to-valley height is determined according to DIN 4768 (in the October 1970 version). The peak-to-valley height R.sub.z is then the arithmetic mean calculated from the individual peak-to-valley height values of five mutually adjacent individual measurement lengths.
Graining is, inter alia, carried out in order to improve the adhesion of the reproduction layer to the support and to improve the water/ink balance of the printing form which results from the printing plate upon irradiation (exposure) and developing. By irradiating and developing (or decoating, in the case of electrophotographically-working reproduction layers), the ink-receptive image areas and the water-retaining non-image areas (generally the bared support surface) in the subsequent printing operation, are produced on the printing plate, and thus the actual printing form is obtained. The final topography of the aluminum surface to be grained is influenced by various parameters. By way of example, the following passages from the literature supply information about these parameters:
The paper "The Alternating Current Etching of Aluminum Lithographic Sheet", by A. J. Dowell, published in Transactions of the Institute of Metal Finishing, 1979, Vol. 57, pages 138 to 144, presents basic comments on the graining of aluminum in aqueous solutions of hydrochloric acid, based on variations of the following process parameters and an investigation of the corresponding effects. The electrolyte composition is changed during repeated use of the electrolyte, for example, in view of the H.sup.+ (H.sub.3 O.sup.+) ion concentration (measurable by means of the pH) and in view of the Al.sup.3+ ion concentration, with influences on the surface topography being observed. Temperature variations between 16.degree. C. and 90.degree. C. do not show an influence causing changes until temperatures are about 50.degree. C. or higher, the influence becoming apparent, for example, as a significant decrease in layer formation on the surface. Variations in graining time between 2 and 25 minutes lead to an increasing metal dissolution with increasing duration of action. Variations in current density between 2 and 8 A/dm.sup.2 result in higher roughness values with rising current density. If the acid concentration is in a range from 0.17 to 3.3% of HCl, only negligible changes in pit structure occur between 0.5 and 2% of HCl, whereas below 0.5% of HCl, the surface is only locally attacked, and at the high values, an irregular dissolution of aluminum takes place. An addition of SO.sub.4.sup.2- ions or Cl.sup.- ions in the form of salts (e.g., by adding Al.sub.2 (SO.sub.4).sub.3 or NaCl) can also influence the topography of the grained aluminum. Rectification of the alternating current shows that both half-wave types are necessary to obtain a uniform graining.
The use of hydrochloric acid as an electrolyte in the graining of aluminum substrates is thus to be considered as being basically known in the art. A uniform graining can be obtained, which is appropriate for lithographic plates and is within a useful roughness range. In pure hydrochloric acid electrolytes adjustment of an even and uniform surface topography is difficult and it is necessary to keep the operating conditions within very close limits.
The influence of the electrolyte composition on the quality of graining is, for example, also described in the following publications:
German Offenlegungsschrift No. 22 50 275 (=British Patent Specification No. 1,400,918) specifies aqueous solutions containing from 1.0 to 1.5% by weight of HNO.sub.3 or from 0.4 to 0.6% by weight of HCl and optionally from 0.4 to 0.6% by weight of H.sub.3 PO.sub.4, for use as electrolytes in the graining of aluminum for printing plate supports, by means of an alternating current,
U.S. Pat. No. 4,072,589 mentions aqueous solutions containing from 0.2 to 1.0% by weight of HCl and from 0.8 to 6.0% by weight of HNO.sub.3 as electrolytes in the graining of aluminum employing an alternating current.
Additives used in the HCl electrolyte serve the purpose of preventing an adverse local attack in the form of deep pits. The following additives to hydrochloric acid electrolytes are, for example, described:
in U.S. Pat. No. 4,172,772: monocarboxylic acids, such as acetic acid,
in U.S. Pat. No. 3,963,594: gluconic acid,
in European Patent Application No. 0 036 672: citric acid and malonic acid and
in U.S. Pat. No. 4,052,275: tartaric acid.
All these organic electrolyte components have the disadvantage of being electrochemically instable and decomposing in the case of a high current load (voltage).
Inhibiting additives, for example, phosphoric acid and chromic acid as described in U.S. Pat. No. 3,887,447 or boric acid as described in U.S. Pat. No. 3,980,539 have the disadvantage that there is often a local breakdown of the protective effect and individual, particularly pronounced pits can form in these places.
Japanese Patent Application No. 91 334/78 describes graining by means of an alternating current in a composition comprising a combination of hydrochloric acid and an alkali-metal halide, to produce a lithographic support material.
U.S. Pat. No. 3,632,486 and No. 3,766,043 describe graining by means of a direct current, for example, for decorative panellings, using dilute hydrofluoric acid, whereby the aluminum is switched such that it forms the anode.
German Pat. No. 120 061 describes a treatment for generating a hydrophilic layer by the application of electric current, which treatment can also be performed in hydrofluoric acid.
Japanese Patent Application No. 93 108/78 describes the production of a capacitor film; in the process, graining is first carried out in an electrolyte comprising from 0.3 to 1.5% of hydrochloric acid and from 15 to 25% of ammonium acetate using an alternating current (at 200 to 400 C/dm.sup.2), and electrolysis is then continued in HCl using a pulsed current.
Japanese Patent Application No. 105 471/78 claims 0.3 to 1.5% of HNO.sub.3 and 1 to 3.0% of citric acid, in addition to 15 to 25% of ammonium acetate.
However, a treatment of this kind in electrolyte systems with a pH exceeding 4.5 leads to surface structures which are coarsely pitted and/or do not show an overall graining and which hence are entirely unsuited for lithographic purposes. Contrary to surface enlargement which is desired for the application in capacitors, roughening of printing plate supports serves to produce layer anchoring and water/ink balance and must therefore be very homogeneous and free from pits.
The use of acetylacetone in ordinary metal cleaning agents is, for example, described in German Offenlegungsschrift No. 19 26 809. The object of the present invention is, however, to produce a support material which is suitable for lithographic purposes and, therefore, must exhibit an extremely homogeneous surface topography.
Another known possibility for improving the uniformity of electrochemical roughening comprises a modification of the type of electric current employed, including, for example,
using an alternating current, in which the anodic voltage and the anodic coulombic input are higher than the cathodic voltage and the cathodic coulombic input, according to U.S. Pat. No. 4,087,341, the anodic half-cycle period of the alternating current being generally adjusted to be less than the cathodic half-cycle period; this method is, for example, also referred to in U.S. Pat. No. 4,301,229, German Offenlegungsschrift No. 30 12 135 (=published UK Patent Application No. 2,047,274) or U.S. Pat. No. 4,272,342,
using an alternating current, in which the anodic voltage is markedly increased compared with the cathodic voltage, according to U.S. Pat. No. 3,193,485,
interrupting the current flow for 10 to 120 seconds and re-applying current for 30 to 300 seconds, using an alternating current and, as the electrolyte, an aqueous solution of 0.75 to 2.0 N HCl, with the addition of NaCl or MgCl.sub.2, according to British Patent No. 879,768. A similar process comprising an interruption of current flow in the anodic or cathodic phase is also disclosed in German Offenlegungsschrift No. 30 20 420 (=U.S. Pat. No. 4,294,672).
The aforementioned methods may lead to relatively uniformly grained aluminum surfaces, but they sometimes require a comparatively great equipment expenditure and, in addition, are applicable only within closely limited parameters.